Hot tubs and spa units have become a popular addition to many households. The water in these units is generally heated by natural gas or electricity. Due to the increasing cost of both sources of energy, the economics of maintaining such units has become prohibitive. At the same time, the relative abundance of wood and/or coal in many areas of the country presents a reasonable alternative to heating such units.
Heating tanks of water by direct contact of the water with the hot combustion gases or a surface heated by hot combustion gases is well known. Submerged heaters of cast iron or sheet galvanized iron for heating water in stock watering troughs to prevent freezing are described in U.S. Pat. Nos. 413,039; 414,398; 908,182; 1,237,994; 1,239,925; 1,397,831; 1,604,049; and 1,700,285. Portable bathtubs employing oil heaters are described in U.S. Pat. Nos. 2,005; 1,315,987; and 1,450,444.